Biocompatible semiconductor quantum dot (QD) probes with extended plasma circulating times have been developed for cancer imaging in living animals. The structural design involves encapsulating luminescent QDs with a triblock copolymer, and linking this amphiphilic polymer to multiple poly(ethylene glycol) (PEG) molecules. In vitro histology and in vivo imaging studies indicate that the QD probes can be delivered to tumor sites by enhanced permeation and retention. Using both systemic injection of long-circulating QD probes and subcutaneous injection of QD-tagged microbeads, we have achieved sensitive and multicolor fluorescence imaging of cancer cells under in vivo conditions. These results raise new possibilities for ultrasensitive and multiplexed imaging of molecular targets in vivo.